The present invention relates to a projection lens, and in particular, relates to a projection lens suitable to be applied in a projection television, etc.
There is already known so-called a projection television apparatus, in which an image displayed on a fluorescent screen of a CRT projection tube is magnified and projected upon a screen through a projection lens. The performances required for such the projection lens increase up every year, which determine the picture quality of the projection television apparatus. Under such situation, there is described an invention for improving the performances of image formation of the projection lens, for example, in the following Patent Document 1.
With the conventional technology discloses in the Patent Document 1, there is already know a projection lens of a hybrid method (combining a glass lens and a resin lens therein) within the projection lens constituted by five lens units or groups, wherein a third lens unit is made of a glass material having any value within a range from 50 to 75 of Abbe number, while a first lens unit and a fourth lens unit are made of a resin or plastic material have any number within a range from 20 to 35 of Abbe number and also a second lens unit and a plastic lens within a fifth lens unit have any number within a range from 50 to 60 of Abbe number, thereby achieving the high performances of image formation.
Herein, a faceplate of the CRT projection tube, being a constituent element of the fifth lens group, has a spherical surface of 350 mm of the curvature radius in the configuration of the fluorescent screen, thickness of 14.1 mm at an optical axis, and refractive index of 1.562 in the material thereof.
On the other hand, various efforts were also made for increasing up an original picture itself in quality thereof, which is displayed on the CRT projection tube; such as, adopting a crystalline transparent glass material into the faceplate, for example, thereby developing a CRT projection tube of achieving high resolution (8 Lp/mm), high brightness, and a long lifetime.
The spherical surface of the CRT projection tube having such high resolution is flat on the surface thereof, and further the faceplate thereof is very thin in the thickness. Also, the material thereof has high refractive index, such as, from 1.715 to 1.833; therefore, it is impossible to deal with such tube with only applying the ideas, which are adopted into and extended from such the conventional projection lens.
Patent Document 1: Japanese Patent Laying-Open 2000-81569 (2000).